Photoluminescence of Nanoporous GaN Films Prepared by Electrochemical Etching

Nanoporous （NP） CaN is prepared by electrochemical etching on a CaN epilayer grown on a sapphire substrate by metal-organic chemical vapor deposition. Scanning electron microscopy reveals that the average pore diameter and inter-pore spacing are approximately 25 and 45 nm, respectively. The photoluminescence （PL） spectra show that in contrast to the initial as-grown CaN epilayer, the NP CaN exhibits a high near-band-edge UV intensity, significant relaxation of compressive strain, and a lower yellow luminescence intensity. Both the line shape and line width of the PL spectra are almost the same for these two samples. The high quality of the NP CaN can be explained by the enhancement of the PL extraction efficiency and the decrease of impurity and defect density after etching.     

Influence of Phase Transition of Starting Materials on Growth of GaN Nanomaterials by CVD

Ground by mechanical ball milling under certain conditions, β-Ga2O3 powders can transit to ε-Ga2O3 ones. As starting materials, Ga2O3 powders treated by different methods are used to prepare GaN nanomaterials. It is found that the morphologies of GaN nanomaterials are quite different due to the phase transition of Ga2O3 from β-Ga203 to ε-Ga203.     

High-Pressure and High-Temperature Behaviour of Gallium Oxide

High-temperature and high-pressure behaviours of β-Ga2O3 powder are studied by energy-dispersive x-ray diffrac- tion in a diamond anvil cell （DAC）. It is found that the phase transition from the monoclinic β-Ga2O3 to the trigonal α-Ga2O3 occurs at around 19.2 GPa under cold compression. By heating the powder to 2000 K at 30 GPa, we confirm that α-Ga2O3 is the most stable structure at the high pressure. Furthermore, the structural transition from β-Ga2O3 to α-Ga2O3 is irreversible. After laser heating, the recrystallized Ga2O3 has a preferable （012） orientation. This interesting behaviour is also discussed.     

Transformation from β-Ga2O3 to GaN Nanowires via Nitridation

Element doping is an important way to modify the properties of semiconductor materials. In our previous work, it was found that nitrogen-doping in β-Ga2O3 nanowires can induce a novel luminescence emission （around 740 nm） caused by generation of acceptor levels at the middle of the band gap of the β-Ga2O3 nanowires. Here we report that further heavy doping of nitrogen can transform the β-Ga2O3 nanowires completely into wurtzite structured GaN nanowires. Transmission electron microscopy （TEM）, x-ray diffraction （XRD） and Raman spectrum are used to evaluate the transition process. Both XRD and Raman analysis reveal that the monoclinic β-Ga2O3 nanowires start phase transformation at a temperature around 850℃ towards wurtzite structured GaN. Our results will be very helpful to profound our understanding of the doping induced effects and phase transformation in semiconductor compounds.     

Electronic structure and optical properties of Sn_(2x)Ga_(2(1-x))O_3 compounds

Band structure, density of states, electron density difference, and optical properties of intrinsic β-Ga2O3 and Sn2xGa2(1-x)O3 (x= 3.125%-6.25%) compounds are studied using first-principle calculations based on the density functional theory. The anisotropic optical properties are investigated by means of the complex dielectric function, which are explained by the selection rule of band-to-band transitions. All the calculation results indicate that the conductivity of Sn2xGa2(1-x)O3 is super to β-Ga2O3, and ...     

Growth of a-Plane InN Film and Its THz Emission

We report the growth of a-plane InN on an r-plane sapphire substrate by plasma-assisted molecular-beam epitaxy. It is found that the a-plane InN is successfully grown by using a CaN buffer layer, which has been confirmed by reflection high-energy electron diffraction, x-ray diffraction and Raman scattering measurements. The Hall effect measurement shows that the electron mobility of the as-grown a-plane InN is about 406 cm^2/V·s with a residual electron concentration of 5.7 × 10^18 cm^-3. THz emission from the a-plane InN film is also studied, where it is found that the emission amplitude is inversely proportional to the conductivity.     

Nanoscale superconductivity of ?-Ga islands grown by molecular beam epitaxy

We report on the preparation and superconductivity of metastable γ-Ga islands on Si(111) substrate. The Ga grows in a typical Volmer-Weber mode at a low temperature of 110 K, resulting in formation of Ga nanoislands at various sizes with the identical γ-phase. In-situ low temperature scanning tunneling spectra reveal quantized electronic states in ultrathin Ga islands. It is found that both the lateral size and thickness of the Ga islands strongly suppress the superconductivity. Due to substantial surface energy contribution, the transition temperature Tc scales inversely with the island thickness and the minimum thickness for the occurrence of superconductivity is calculated to be two monolayers.     

Effects of N-doping concentration on the electronic structure and optical properties of N-doped β-Ga2O3

The electronic structures and the optical properties of N-doped β-Ga2O3 with different N-doping concentrations are studied using the first-principles method.We find that the N substituting O（1） atom is the most stable structure for the smallest formation energy.After N-doping,the charge density distribution significantly changes,and the acceptor impurity level is introduced above the valence band and intersects with the Fermi level.The impurity absorption edges appear to shift toward longer wavelengths with an increase in N-doping concentration.The complex refractive index shows metallic characteristics in the N-doped β-Ga2O3.     

Effects of N-doping concentration on the electronic structure and optical properties of N-doped β-Ga₂O₃

The electronic structures and the optical properties of N-doped β-Ga2O3 with different N-doping concentrations are studied using the first-principles method.We find that the N substituting O(1) atom is the most stable structure for the smallest formation energy.After N-doping,the charge density distribution significantly changes,and the acceptor impurity level is introduced above the valence band and intersects with the Fermi level.The impurity absorption edges appear to shift toward longer wavelengths with an increase in N-doping concentration.The complex refractive index shows metallic characteristics in the N-doped β-Ga2O3.     

Elimination of turned dendrite in laser multilayer deposition of Rene88DT superalloy on DD3 single crystal

Laser multilayer deposition of Rene88DT superalloy on DD3 single-crystal substrate is conducted.The influences of the crystal orientation of the substrate and the profile of the solid/liquid interface of the molten pool on the deposited microstructure are investigated.A unique strategy is proposed by adjusting the angle between the substrate surface and the substrate crystal orientation.This approach prevents the formation of the turned dendrite,thus obtaining a fully directional microstructure in the deposits.     

Investigation of optical emission spectroscopy in diamond chemical vapor deposition by Monte Carlo simulation

The optical emission spectra(atomic hydrogen(H_α,H_β,H_γ),atomic carbon C(2p3s→2p~2:λ=165.7 nm)and radical CH(A~2Δ→X~2Π:λ=420-440 nm))in the gas phase process of the diamond film growth from a gas mixture of CH4 and H_2 by the technology of electron-assisted chemical vapor deposition (EACVD)have been investigated by using Monte Carlo simulation.The results show that the growth rate may be enhanced by the substrate bias due to the increase of atomic hydrogen concentration and the mean temperature of electrons.And a method of determining the mean temperature of electrons in the plasma in-situ is given.The strong dependence on substrate temperature of the quality of diamond film mainly attributes to the change of gas phase process near the substrate surface.     

Refractive index inhomogeneity of LaF3 film at deep ultraviolet wavelength

It is well known that the optical property of an optical thin film can be influenced by even small inho- mogeneity of refractive index （RI）. In order to investigate the RI inhomogeneity of LaF3 single layer in deep ultraviolet （DUV） range, single-layer LaF3 samples deposited on fused silica and CaF2 substrates are prepared by resistive heating evaporation at different deposition temperatures. The reflectance and transmittance spectra of LaF3 film samples are measured with a spectrophotometer, and used to calculate the RI inhomogeneity. The experimental results show that no RI inhomogeneity of LaF3 film is observed when deposited on CaF2 substrate, while negative RI inhomogeneity is presented when deposited on fused silica substrate. The level of inhomogeneity is affected by the substrate temperature, which decreases with the increasing substrate temperature from 250 to 400 ℃.     

Study on nitridation processes of β-Ga_2O_3 single crystal

Nitridatedβ-Ga_2O_3(100)substrate was investigated as the substrate for GaN epitaxial growth.The effects of nitridation temperature and surface roughness ofβ-Ga_2O_3 wafers on the formation of GaN were studied.It was found that the most optimized nitridation temperature was 900°C,and hexagonal GaN with preferred orientation was produced on the well-polished wafer.The nitridation mechanism was also discussed.     

Investigation of optical emission spectroscopy in diamond chemical vapor deposition by Monte Carlo simulation

The optical emission spectra(atomic hydrogen(Hα,Hβ,Hγ),atomic carbon C(2p3s→2p2:λ=165.7 nm) and radical CH(A2△→X2П:λ=420-440 nm))in the gas phase process of the diamond film growth from a gas mixture of CH4 and H2 by the technology of electron-assisted chemical vapor deposition (EACVD)have been investigated by using Monte Carlo simulation.The results show that the growth rate may be enhanced by the substrate bias due to the increase of atomic hydrogen concentration and the mean temperature of electrons.And a method of determining the mean temperature of electrons in the plasma in-situ iS given.The strong dependence on substrate temperature of the quality of diamond film mainly attributes to the change of gas phase process near the substrate surface.     

Investigation of β-Ga_2O_3 films and β-Ga_2O_3/GaN heterostructures grown by metal organic chemical vapor deposition

In this work,(-201) β-Ga_2O_3 films are grown on GaN substrate by metal organic chemical vapor deposition(MOCVD). It is revealed that the β-Ga_2O_3 film grown on GaN possesses superior crystal quality, material homogeneity and surface morphology than the results of common heteroepitaxial β-Ga_2O_3 film based on sapphire substrate. Further, the relevance between the crystal quality of epitaxial β-Ga_2O_3 film and the β-Ga_2O_3/GaN interface behavior is investigated. Transmission electron microscopy result indicates that the interface atom refactoring phenomenon is beneficial to relieve the mismatch strain and improve the crystal quality of subsequent β-Ga_2O_3 film. Moreover, the energy band structure of β-Ga_2O_3/GaN heterostructure grown by MOCVD is investigated by X-ray photoelectron spectroscopy and a large conduction band offset of 0.89 eV is obtained. The results in this work not only convincingly demonstrate the advantages of β-Ga_2O_3 films grown on GaN substrate, but also show the great application potential of MOCVD β-Ga_2O_3/GaN heterostructures in microelectronic applications.     

Controlling the surface buckling wrinkles by patterning the material system of hard-nano-film/soft-matter-substrate

Wrinkling and buckling of nano-films on the compliant substrate are always induced due to thermal deformation mismatch.This paper proposes effective means to control the surface wrinkling of thin film on the compliant substrate,which exploits the curvatures of the curve cracks designed on the stiff film.The procedures of the method are summarized as:1)curve patterns are fabricated on the surface of PDMS(Polydimethylsiloxane)substrate and then the aluminum film with the thickness of several hundred nano-meters is deposited on the substrate;2)the curve patterns are transferred onto the aluminum film and lead to cracking of the film along the curves.The cracking redistributes the stress in the compressed film on the substrate;3)on the concave side of the curve,the wrinkling of the film surface is suppressed to be identified as shielding effect and on the convex side the wrinkling of the film surface is induced to be identified as inductive effect.The shielding and inductive effects make the dis-ordered wrinkling and buckling controllable.This phenomenon provides a potential application in the fabrication of flexible electronic devices.     

Deposition of Diamond-Like carbon Films by High-Intensity Pulsed Ion Beam Ablation at Various Substrate Temperatures

Diamond-like carbon (DLC) films have been deposited on to Si substrates at substrate temperatures from 255℃ to 400O℃ by a high-intensity pulsed-ion-beam (HIPIB) ablation deposition technique. The formation of DLC is confirmed by Raman spectroscopy. According to an x-ray photoelectron spectroscopy analysis, the concentration of sp^3 carbon in the films is about 40% when the substrate temperature is below 300℃ C. With increasing substratetemperature from 25℃ to 400℃, the concentration of sp^3 carbon decreases from 43% to 8%. In other words,sp3 carbon is graphitized into sp^2 carbon when the substrate temperature is above 300℃. The results of xray diffraction and atomic force microscopy show that, with increasing the substrate temperature, the surface roughness and the friction coefficient increase, and the microhardness and the residual stress of the films decrease.     

Thermal stress reduction of GaAs epitaxial growth on V-groove patterned Si substrates

We investigate the thermal stresses for GaAs layers grown on V-groove patterned Si substrates by the finite-element method. The results show that the thermal stress distribution near the interface in a patterned substrate is nonuniform,which is far different from that in a planar substrate. Comparing with the planar substrate, the thermal stress is significantly reduced for the Ga As layer on the patterned substrate. The effects of the width of the V-groove, the thickness, and the width of the SiO₂ mask on the thermal stress are studied. It is found that the SiO₂ mask and V-groove play a crucial role in the stress of the Ga As layer on Si substrate. The results indicate that when the width of V-groove is 50 nm, the width and the thickness of the SiO₂ mask are both 100 nm, the Ga As layer is subjected to the minimum stress. Furthermore,Comparing with the planar substrate, the average stress of the Ga As epitaxial layer in the growth window region of the patterned substrate is reduced by 90%. These findings are useful in the optimal designing of growing high-quality Ga As films on patterned Si substrates.     

Synthesis of GaN Nanorods by Ammoniating Ga2O3/ZnO Films

Large quantities of CaN nanorods are successfully synthesized on Si（111） substrates by ammoniating the films of Ga2O3/ZnO at 950℃ in a quartz tube. The structure, morphology and optical properties of the as-prepared CaN nanorods are studied by x-ray diffraction, scanning electron microscopy, high-resolution transmission electron microscopy, Fourier transform infrared spectroscopy, and photoluminescence. The results show that the CaN nanorods have a hexagonal wurtzite structure with lengths of several micrometres and diameters from 80 nm to 300hm, which could supply an attractive potential to harmonically incorporate future GaN optoelectronic devices into Si-based large-scale integrated circuits. The growth mechanism is also briefly discussed.     

Micro—structural analysis of YBa2Cu3O7—x thin films grown on different substrates by X—ray techniques

YBa2Cu3Ox(YBCO) thin films grown on different substrates with and/or without Eu2CuO4(ECO) buffer layer were investigated by X-ray wide angle diffraction,reflection,diffuse scattering and topography.Theresults show that for the yttria stabilized ZrO2(YSZ) substrate,the presence of an ECO buffer layer improves the crystalline quality of the YBCO film,while a negative effect is observed for the SrTiO3(STO) substrate.The lateral correlation length for a sample grown on a YSZ substrate with ECO buffer Layer is much greater than grown on an STO subetrate.The STO substrate used has mosaic structure.2001 Elsevier Science B.V.All rights reserved.